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The present invention generally relates to centrifuges and in particular to a centrifuge enabling automatic discharge of solids that accumulate during separation.
Many different types of centrifugal separators are known for separating heterogeneous mixtures into components based on specific gravity. A heterogeneous mixture, which may also be referred to as feed material or feed liquid, is injected into a rotating bowl of the separator. The bowl rotates at high speeds and forces particles of the mixture, having a higher specific gravity, to separate from the liquid by sedimentation. As a result, a dense solids cake compresses tightly against the surface of the bowl, and the clarified liquid, or xe2x80x9ccentratexe2x80x9d, forms radially inward from the solids cake. The bowl may rotate at speeds sufficient to produce forces 20,000 times greater than gravity to separate the solids from the centrate.
The solids accumulate along the wall of the bowl, and the centrate is drained off. Once it is determined that a desired amount of the solids has been accumulated, the separator is placed in a discharge mode. In one such discharge mode, a scraper blade extending the length of the rotating bowl is placed in a scraping position against the separator wall and the bowl is rotated at a low scraping speed. Then, a radial-motion scraper scrapes the solids from the sides of the bowl, and they fall toward a solids collecting outlet. However, such a radial-motion scraper does not effectively remove wet or sticky solids which may have a consistency like that of peanut butter. In such instances, the sticky solids remain stuck on the scraper blades or fall from the wall and then reattach to the blades before reaching the collecting outlet. As a result, the solids recovery yield is reduced and the remaining solids undesirably contaminate the separator.
An additional important consideration in the design of centrifugal separators is to minimize vibration and other ill effects of operation at high rotational speeds. The separator bowl and its mounting structure form a mechanical unit having inherent resonant or xe2x80x9ccriticalxe2x80x9d speeds which are preferably avoided during operation. An additional consideration is potential for axial movement of the separator bowl, for example in the presence of imbalance or the motion of liquid axial waves in the bowl, which can result in unstable operation.
In accordance with the present invention, a centrifugal separator is disclosed that includes features addressing the shortcomings of existing centrifugal separators, especially shortcomings associated with solids recovery and mechanical instability.
In one aspect, the disclosed centrifugal separator provides for automatic discharge of solids by means of either an axial-motion scraper or a piston/extrusion assembly with exchangeable parts, having variable speed operation for greater versatility. The axial-motion scraper is used with hard-packed or friable solids, and includes an integral feed liquid accelerator and feed holes. The scraper blades flex outwardly under high centrifugal force to lock the scraper in place against the bowl. This provides a rigid or fixed end condition for the lower end of the scraper shaft to allow for high critical speed of the shaft. The scraper provides less surface area for solids to stick to, and can be used in conjunction with relatively long separator bowls.
The piston/extrusion assembly is used for pasty, sticky solids that can be extruded. A centrate valve at the top of the bowl is used to enable the centrate (separated liquid) to be discharged during a feed mode of operation, and then to close off the top of the bowl for a solids discharge mode of operation. The assembly further includes a piston that sits at the bottom of the bowl during the feed mode of operation. The piston has an integral feed accelerator and feed holes through which the feed liquid passes. These holes also provide exit paths for the solids during the extrusion that takes place in the solids discharge mode of operation. The piston/extrusion assembly can be used with sticky solids that other existing centrifuges cannot discharge efficiently, and provides for nearly complete removal of the solids, which is desirable for example when the solids contain valuable materials.
In another aspect, the disclosed centrifugal separator includes a separator bowl suspension that employs a short, stiff spindle and a spherically mounted bearing housing. Conceptually, the arrangement is analogous to a vertical rotating beam with a simply supported upper end. This arrangement has a very high critical speed as compared to existing centrifuges. It is possible to achieve a critical speed greater than the highest operating speed, so that the critical speed is not encountered during operation. The spherically mounted bearing housing restrains axial motion of the separator bowl and provides for stable operation at higher speeds than prior mounting arrangements.
In yet another aspect, the disclosed centrifugal separator employs a half-ball-shaped solids discharge valve at the bottom of the case. The discharge valve incorporates respective passages for the feed liquid and for residual liquid being drained from the bowl. The valve rotates between a closed position in which the bottom of the case is closed except for the openings to and from the feed liquid and residual liquid passages, and an open position in which solids being discharged from the separator bowl are able to fall out of the bottom of the case. This arrangement is generally more compact than prior art arrangements for discharge valves, and can be used in sanitary and/or clean-in-place applications.
Other aspects, features, and advantages of the present invention will be apparent from the Detailed Description of the Invention that follows.